Sensors can be found everywhere in today's society—heat sensors are utilized in a common thermostat to activate/deactivate heating and cooling units. Light sensors can be used to govern when to turn on and off automobile headlights (e.g., if the sensor does not receive a predetermined minimum amount of light, a control unit will activate headlights). Even an alarm clock can be thought of as a time sensor, wherein upon sensing a preset time a signal is output from the clock. Dynamic sensing of torque and strain in a rotating shaft would be beneficial towards improving design and analysis of machinery, as well as allowing for torque and strain of a rotating shaft to be utilized as a control parameter in a control system(s). Dynamic torque information also provides an important capability to perform real-time diagnostics for rotating machinery and diagnose mechanical faults thereby predicting and mitigating catastrophic failure. Robust, affordable, lightweight sensing systems for obtaining data regarding a rotating shaft (specifically torque), however, are presently non-existent.
Rotating shafts driving a load are susceptible to torsion strain. Continuous strain on the shaft will eventually result in machine inefficiency and/or shaft malformation and/or breakage. Dynamic torque (strain) fluctuations can cause mechanical and fatigue damage as well as accelerate machine failure. A sensor used to measure torsion strain (torque) on a shaft is therefore be desirable. However, conventional torque sensors are large, costly, and heavy devices. For example, a conventional torsion strain sensing system can weigh approximately 1500 pounds if one desired to measure torque found in a tail rotor of a helicopter. Furthermore, the conventional shaft torque sensing system in the example above would cost at least five thousand dollars, be susceptible to failure due to shock loads, and take up valuable aircraft space and payload capacity. Conventional sensing systems that consider angular displacement difference between two ends of a rotating shaft typically involve costly and complex optics and mechanical interconnect equipment.
Using a property of photo-elasticity in conjunction with optical sensors to measure torque on a rotating shaft is one prior art method to mitigate disadvantages of conventional torque measuring systems. Optical sensing systems are desirable because they are compact and lightweight in comparison to conventional electric or magnetic sensors, and have significantly greater immunity to electromagnetic interference as compared to many conventional systems. Furthermore, optical sensing systems can be produced inexpensively and allow for quick replacement/repair of the system. Lastly, optical sensing systems can provide high frequency torque signals unobtainable with conventional torque sensor(s).
The property of photo-elasticity has been used to measure strain on materials for over fifty years. The method of determining strain relies on the birefringence property exhibited by transparent plastics (i.e., light waves are split into two parallel waves which are polarized perpendicularly). In particular, the phenomenon of load-induced birefringence is utilized where a material becomes birefringence under influence of external loading. In practice, polarized light is delivered into a photo-elastic material wherein a strain in the photo-elastic material is encountered. The normally incident polarized light is split into two components along principal strain directions in a plane perpendicular to direction of light propagation and are transmitted only along these planes through a photo-elastic material. The velocities of light transmission along these directions is directly proportional to magnitude of respective principal strains. The light is then passed through a second polarizer, which results in a two-dimensional light-intensity pattern—the strain on the material can be deduced by inspection of the resulting light-intensity pattern.
FIG. 1 illustrates an exemplary cross-sectional view of a prior art optical sensing system 100 which can be employed to measure torque on a rotating shaft. The sensing system 100 includes a light emitting component 102, a capturing component 104, and strips of photo-elastic material 106. The strips of photo-elastic material 106 encircle a rotating shaft 108, and interiors of the strips of photo-elastic material 106 are coated with a reflective substance such as an aluminum filled epoxy. In operation, the light emitting component 102 releases a beam of light into a strip of photo-elastic material 106. The light reflects off of a reflective interior surface of the strip of the photo-elastic material 106, and back out through the photo-elastic material and into the capturing component 104. A light-intensity pattern is visible from the light exiting the strips of photo-elastic material 106. The light-intensity pattern is a spectrum of colors known in the art as a fringe pattern, wherein each fringe is comprised of a band of colors. The number of fringes, narrowness and proximity of the fringes in the fringe pattern indicate the amount of strain experienced in the photo-elastic material, and therefore indicate the amount of strain on the rotating shaft 108.
The subject invention as described below provides for significant improvements over the aforementioned prior art systems and addresses unmet needs with respect to the amount of shaft data desired in connection with rotating shafts.